In the field of optical telecommunications, it is desirable to increase the data rate conveyed by light, and one of the means used for achieving that object consists in using light sources that are more and more monochromatic. Distributed feedback laser diodes have narrow spectrum width and have appeared as particularly good light sources.
In laser diodes of this type, the light feedback necessary for the lasing phenomenon comes from Bragg backscattering that results from periodic disturbance of the refractive index of a region in the structure of the diode in which the lightwave propagates. To do this, a diffraction grating is made that includes patterns distributed along the structure, said patterns generally extending transversely to the propagation direction of the light.
In conventional distributed feedback laser diodes, the diffraction grating provides constant amplitude coupling between light propagating in one direction (the "go" wave) and light propagating in the opposite direction (the "return" wave).
At a high level of emitted power, instabilities are observed in the operation of conventional laser diodes which give rise to the appearance of longitudinal secondary modes that are superposed on the main mode, with the drawback of spoiling the performance of such laser diodes, in particular by broadening the emission spectrum and by degrading the linearity of the emitted power response as a function of injected current. This instability phenomenon known by the term "hole-burning" is the result mainly of non-uniformity in the electrical field of the lightwave along the structure.
Various structures have been proposed to improve the performance of conventional laser diodes, and to limit the "hole-burning" phenomenon.
In an article entitled "Long cavity, multiple-phase shift, distributed feedback laser for linewidth narrowing" published in Electronics Letters Vol. 25, No. 10, pp. 629-630, 1989, Ogita et al. propose implementing a plurality of phase shifts along the grating. However, the Ogita study is restricted to short laser structures, about 500 .mu.m long, since the instabilities due to "hole-burning" tend to increase with the length of the structure. Unfortunately, it appears to be advantageous to implement structures of greater length since the monochromaticity of the emitted spectrum improves considerably when the length of the structure is increased.
In an article entitled "Corrugation-pitch-modulated phase-shifted DFB laser", published in IEE Photonics Technology Letters, Vol. 1, No. 8, August 1989, Okai et al. describe a structure having a length of 1.2 mm. Okai proposes modulating the period of the grating to guarantee stable operation. However, although the "hole-burning" phenomenon is considerably reduced, it is not completely eliminated.
Other authors have proposed different approaches to counter this problem of "hole-burning". One such article entitled "A new DFB-laser diode with reduced spatial hole-burning" published in IEE Photonics Technology Letters, Vol. 2, No. 6, June 1990, by Morthier et al. shows that it is theoretically possible to improve the distribution of the electric field of the lightwave within the structure and thereby reduce the "hole-burning" phenomenon by varying coupling amplitude, such that the coupling is at a minimum in the center of the structure.
However, the Morthier study is limited to short structures, about 300 .mu.m long, delimited by faces with anti-reflection treatment. Morthier observes that coupling modulation is accompanied by a phase shift of the lightwave in the structure which prevents the laser from oscillating at the Bragg frequency, with the drawback of leading to low rejection of secondary modes. The author neither proposes nor suggests any solutions for attempting to remedy that drawback.